Lessons From Deep Space: Why Pioneer Is Important

NASA's Voyager spacecraft is not the only important deep-space mission to remember. The Pioneer spacecraft and the Pioneer Anomaly are part of a classic engineering story about good design.

The most dramatic-looking feature of the Pioneer spacecraft was a 2.74 m diameter parabolic dish used to stay in communications with earth. One of its tasks was to take a 2.1-GHz S-band carrier signal transmitted from earth and coherently up-convert it by a factor of 240/210 to about 2.29 GHz and retransmit it back.

Since the received frequency stability was as accurate as the earth-station's frequency stability, small frequency shifts on the order of 1-mHz could be detected. This is a frequency shift of a part in 1 trillion over a few hours.

Precision measurements of frequency shifts received on earth could be interpreted as the Doppler shift in the signal. This enabled precision measurement of the spacecraft's line of sight velocity, which could be translated to position and acceleration information.

By 1980, an anomaly had been detected in its motion. The unexplained frequency shift was only a few tens of milliHertz, but it was far above the noise. The spacecraft was slowing down more than expected. The deceleration was about 0.09 nano gees. This is about the same deceleration a car would have from the photon pressure from its high beams.

It could not be accounted for by any known influence, such as the gravitational effects of the sun or planets, the solar wind, cosmic ray fluxes, the precession of the poles of the earth, even residual fuel leakage from the spacecraft. This slight deceleration became known as the Pioneer Anomaly.

It appeared the acceleration was constant and not changing over time. This seemed to confirm the early estimate that the influence of thermal radiation from the radioisotope thermoelectric generators (RTG) was not contributing significantly to the deceleration. If it was a thermal radiation effect, the 88-year half-life of the Plutonium 238 should have contributed to a change in the deceleration as the thermal radiation decreased.

In 1994, out of the blue, Anderson got an email from Michael Nieto, a Los Alamos cosmologist. He was interested in MOND, MOdified Newtonian Dynamics, theories of gravity. Coincidentally, Nieto thought he could use precision measurements of the Pioneer satellite as a test for non-Newtonian gravity effects. He expected anomalous accelerations on the order of the expansion rate slow down of the universe, about 0.07 nano Gees. Here was one possible explanation. Wow! Could this agreement just be a coincidence?

At around the same time, Slava Turyshev joined Anderson to work on the Pioneer Anomaly reviewing all possible spacecraft-related causes for the anomaly.

In 1998, what was known of the Pioneer Anomaly was published to stimulate the scientific community to come up with other explanations.

Almost 1,000 papers were published with explanations for the Pioneer Anomaly, including dark energy, dark matter, string theory, violation of general relativity, extra dimensions, and even variations in the speed of light. The chart below shows the publication rate in papers just posted on arXiv over the years.

But Anderson's team was not willing to let go of possible conventional explanations. They managed to find more historical Doppler data, some on old magnetic tape media, stuffed in moldy cardboard boxes under a staircase at JPL. With the new data, they increased the accuracy and time span of Pioneer 10 measurements to more than 23 years. In the new data, they saw a change in the deceleration. It was decreasing over time.

My connection to Pioneer 10 was my job as computer operator at the University of Iowa Physics Research Center (PRC, now known as Van Allen Hall) in late '73.

Pioneer 10 was just going past Jupiter and they were in "high data rate" and my job consisted of mounting a tape, starting the job, and then finding something to do for the next 8 hours of my 3rd shift while the program generated 35mm film plots of the radiation and magnetometer readings.

The title photo on my web site http://www.rostenbach.com shows me (and me) in front of the Univac 418 computers that did the processing. The left hand computer, the UNIVAC 418 model II (18 bit unary math processor with 16 K [of 18 bit words] of core memory at ½ MHz) did the bulk of the processing and just to the left of it, in the far back corner is the Xerox microfilm plotter.

Thanks for posting this. Very interesting and very learning to read. I think these kind of articles are the ones for young people to go after a technical study rather than the alpha side. And oh do we need them very very hard.... (here in Europe)

That is one of the more fascinating aspects of all space travel to me right now. Our computational tech is changing so much faster than our ability to travel quickly that there is this massive gap created any time we travel far. They have to be proud of their work though, those things have functioned amazingly.

The designers of these craft must be so proud of how well they have done their job, and the amount of information gained from their missions quite apart from the actual data sent back. I've often wondered if they think, "What if these spacecraft had today's technology...?" It takes so long to get there that technology will have advanced considerably by the time they reach their destinations...